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Tracking a green screen with Mocha

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Steven Probets
Tracking a green screen with Mocha
on Mar 26, 2013 at 2:02:08 pm

I need to track a phone with a green screen as the hand holding it is moving. I want to use the track information for a video under the green screen. I am going to key out the green screen to reveal the image behind. With the tracking info it will move with the phone.

The hands touches the screen and I need to know any advice of how to use Mocha to ignore the hand coming into shot and to carry on tracking the screen? Tried finding tutorials but nothing specific to my problem.

Thanks


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Dave LaRonde
Re: Tracking a green screen with Mocha
on Mar 26, 2013 at 2:22:43 pm

It sounds like you're going to shoot hand-held AND green screen. Your green screen should have tracking markers on it.

Here's a dandy source to learn about tracking markers. READ THE TEXT THAT GOES WITH IT:
http://www.hollywoodcamerawork.us/greenscreenplates.html

Depending on the nature of the camera motion, Mocha may not be the best tracking solution for you... but we don't know what you intend to do, so we can't make a better recommendation.

If you intend to use Keylight, remember that it likes the green background to be a little on the dark side. And to help you keep your sanity, keep the background away from the subject a good six feet or more to minimize spill light on the subject.

Dave LaRonde
Former Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Roland R. Kahlenberg
Re: Tracking a green screen with Mocha
on Mar 26, 2013 at 3:28:58 pm

Here are a couple of relevant mocha tutorials -
Tracking with Foreground Occlusion (TIME - 16:00 to 44:35)


Tracking with Foreground Occlusion


HTH
RoRK

Intensive mocha & AE Training in Singapore and Other Dangerous Locations

Imagineer Systems (mocha) Certified Instructor
& Adobe After Effects ACE/ACI (version 7)


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Steven Probets
Re: Tracking a green screen with Mocha
on Mar 27, 2013 at 9:36:08 am

Thanks guys! Very helpful.

I am basically putting an image on a phone screen in Mocha and AFX.

The phone is off screen then on screen. I am having trouble tracking this and then putting a surface on it. When the movement is going off screen my surface gets very 'skewed wiff'.

I tried this tracking tutorial http://forums.creativecow.net/readpost/2/1017437 but it seems the object was tracked off screen automatically. As the phone has motion blur on it mine has not worked like this. Any ideas to track motion objects off screen to ensure the surface is always at the correct angles etc?

Cheers again


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Steven Probets
Re: Tracking a green screen with Mocha
on Mar 27, 2013 at 9:40:42 am

Actually, I think I need to create a tracking area and then possibly key frame the last few frames myself (when the phone goes off screen) but make sure all the points are as they were when the phone is on screen, and just move/rotate them all together?...


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Roland R. Kahlenberg
Re: Tracking a green screen with Mocha
on Mar 27, 2013 at 11:50:14 am

In an ideal situation you would provide your footage so that folks can see exactly what's involved. mocha's tracking engine handles motion blur reasonably well - it all depends on how much motion blur there is and your search parameters in mocha.

As a general rule, when dealing with tracking objects that leave the screen (or get obscured by foreground objects), you have the following options -
1) increase the size of your tracking spline to cover more co-planar area
2) add an additional tracking spline onto a co-planar area
3) move the existing tracking spline onto a co-planar area
4) leave everything as it is and hope no one notices
5) use mocha's adjust track module to fill in the 'tracking data for the missing' frames
6) make the adjustments manually in AE

So ... lots of options for you. YOu should ensure that you export out corner pin data with motion blur (MB) support. In AE, set your comp's MB setting such that Shutter Phase is -1/2 of Shutter Angle - this helps to mimic normal camera settings thus ensuring that your MB is centered on your object. Wrong MB settings will screw up the alignment of your corner pin alignment.

HTH
RoRK

Intensive mocha & AE Training in Singapore and Other Dangerous Locations

Imagineer Systems (mocha) Certified Instructor
& Adobe After Effects ACE/ACI (version 7)


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